I have both, and I gotta say GameCube. Loads are faster, the games are great, it's small as hell, the controller is SO comfortable, and the wireless wavebird controller is fantastic.
look at the gamecube line-up:
Super Monkey Ball 2
The Legend of Zelda
that's probably the best line-up I've ever seen.
X-Box's line-up, as far as my interest goes:
Crazy Taxi 3: High Roller (tomorrow, looks fun as hell)
Medal of Honor: Frontline (though if you wanted that you'd already have it)
Metal Gear Solid: Substance (again, an upgraded version of a great PS2 game)
see, x-box doesn't have THAT many games that are exlusive that are really that guaranteed to be great. Nintendo does. and most of the third party games for box are also on PS2, which you already have.
I say Cube all the way.
oh, and one more thing: Snooch to the noonch!